feat: autotag mixed-bag splitting, search relevance fixes, and multi-library download imports

Autotag: detect "junk drawer" folders with no artist/album consensus
and split them into synthetic per-cluster groups instead of forcing
one match on an unrelated pile of tracks; repair tagging_items rows
left behind by a prior scan orphan-cleanup gap.

Explore: fix an exact artist-name search being drowned out by its own
catalog entries in intent-prior scoring, and prune stale in_library
bookkeeping left behind when a referenced library row is deleted.

Download: fix a multi-library regression where every import failed
with "no library root configured" — the importer resolved the
library root from a legacy single-library config field that nothing
populates in the current multi-library model. It now resolves the
destination library per-request from the request's own library_id.
Also widen the Soulseek search window (12s -> 20s), measured against
real request history to be missing available peers on live queries.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/download/importer.go

@@ -56,11 +56,17 @@ type TagWriterPort interface {
 type LibraryPort interface {
 	// ScanLibrary triggers a rescan so imported files are ingested.
 	ScanLibrary(id int64) error
+
+	// LibraryPath resolves a library's root directory by id.
+	LibraryPath(id int64) (string, error)
 }
 
 // ImportOptions configures how imported files are laid out.
 type ImportOptions struct {
 	// LibraryRoot is the directory imported files are placed under.
+	// Resolved per-request from the request's LibraryID — never a
+	// fixed, app-wide directory, since a user can have several
+	// libraries.
 	LibraryRoot string
 
 	// PathTemplate lays out the destination path.  Supported tokens: